The invention is directed to an apparatus for identifying a subject medium from several possible subject media within a cooking device, such as deep fat fryer. The apparatus contains one subject medium within the cooking device and has a controller, which includes a timer, and generates a conductivity signal. The apparatus also includes a conductivity measuring device which receives the conductivity signal and measures a conductivity value device in response to the conductivity signal. The controller determines the subject medium's composition by comparing the measured conductivity value to predetermined conductivity values for the known group of subject media.